Nanoporous structure formation on the surface of Ge by ion beam irradiation

Abstract

The ion beam induced formation of nanoporous structures on the surface of Ge under controlled conditions of ion dose, flux, and irradiation angle using a focused ion beam was investigated by electron microscopy. The formation of large-scale nanoporous structures on the surface of the Ge specimens with increasing ion dose and increasing flux was observed via nanostructural characterization using scanning electron microscopy and transmission electron microscopy (TEM). Compared with the structure formed under irradiation at 0°, that formed under irradiation at 45° was tilted and large-scale. These results suggest that the number of point defects per unit volume of surface is important for the formation of a nanoporous structure. TEM observations revealed that the nanoporous structural features formed during the initial process were not voids but surface roughness. This mechanism differs from the nanoporous structure formation mechanism of GaSb and InSb. The growth of the nanoporous structure in the vertical direction was promoted until the ion dose was 1 × 1021 ions/m2 or greater. At ion doses greater than 1 × 1021 ions/m2, the growth was saturated. The wall thickness remained almost constant with increasing ion dose.